With a wider bandwidth of a radio communication, a multicarrier communication system in which transmit information is divided into plural frequency bandwidths which is hereinafter called “subcarriers” for communication has been used. Among the multicarrier communication systems, the orthogonal frequency division multiplexing (OFDM) system can realize a high frequency use efficiency with no need of a guard band between the respective adjacent subcarriers by using an orthogonality of signals while improving a resistance to a delay wave by narrowing the bandwidth per subcarrier. The OFDM is employed in wide systems, for example, a worldwide interoperability of microwave access (WiMAX) and a long tern evolution (LTE).
In those communication systems, signals of a fixed pattern, which are hereinafter called “pilot signals”, are inserted into a transmit signal at a transmitter side, and fluctuations of an amplitude and a phase during signal propagation are estimated from an amplitude and a phase of the pilot signals to demodulate a receive signal at a receiver side. A channel estimation is conducted with higher precision as a rate of the pilot signals inserted into the transmit signal is higher, and a communication quality can be enhanced. On the other hand, a rate of data signals is more increased as an insertion ratio of the pilot signals is lower, and a maximum data rate is improved. Therefore, the number of pilot signals to be mapped is reduced as much as possible within the required precision of channel estimation.
FIG. 2 is a diagram illustrating an example of a pilot signal mapping of the LTE system. FIG. 2 illustrates an example disclosed in “3GPP TS 36.211 V8.3.0 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)” (Non Patent Literature 1), which shows a pilot signal mapping when one antenna port is used. In this literature, signals called “reference signals” correspond to pilot signals. FIG. 2 is a schematic diagram in which the axis of abscissa represents an OFDM symbol number, that is, a time axis, and the axis of ordinate represents a subcarrier number, that is, a frequency axis. Each rectangular box represents one modulation symbol such as QPSK or 16QAM. Among those rectangular boxes, gray rectangles of symbols 202 represent the pilot signals, and white rectangles of symbols 201 represent signals such as data signals and control signals except for the pilots. In this example, two pilot signals are mapped per one slot in a time direction, and one pilot is mapped per 6 subcarriers in a frequency direction. In the LTE, positions where the pilot signals are mapped in the time direction are different depending on the number of antennas, but common in all of the cells. On the other hand, the pilot signals are mapped in the frequency direction at different positions depending on the cells. In an example illustrated in FIG. 2, the pilot signals are mapped in a subcarrier n, a subcarrier n+3, a subcarrier n+6, and a subcarrier n+9. However, in another cell, for example, the pilot signals are mapped in a subcarrier n+1, a subcarrier n+4, a subcarrier n+7, and a subcarrier n+10.
FIG. 3 is a diagram illustrating another example of the pilot signal mapping of the LTE system.
FIG. 3 illustrates one example of the pilot signal mapping disclosed in Non Patent Literature 1 like FIG. 2, and is a schematic diagram illustrating the pilot signal mapping in one antenna port when four antenna ports are used. The gray rectangles of the symbols 202 represent the pilot signals, and the white rectangles of the symbols 201 represent non-pilot signals are as in FIG. 2. X-mark rectangles of symbols 203 represent times and frequencies which are not used for signal transmission for the purpose of avoiding collision with the pilot signals of other antenna ports.
In demodulating the respective non-pilot signals, there is used channel information obtained by subjecting times and frequencies, at which the appropriate non-pilot signals are mapped, to interpolation and extrapolation on the basis of channel estimation results using the pilot signals. As illustrated in the schematic diagrams of FIGS. 2 and 3, the pilot signals are smaller in number than the non-pilot signals. Therefore, a disturbance added to one symbol of the pilot signal affects a receive quality of a large number of peripheral non-pilot signals using the channel estimation results of the subject pilot signal. For that reason, a higher receive quality of the pilot signal is required than that of the non-pilot signals. For that reason, for example, Patent literature 1 introduces a technique in which, in order to increase transmission powers of the pilot signals while keeping the total transmission power per hour constant, the transmission powers of the non-pilot signals are averagely lowered, or some of the symbols allocated for transmission of the non-pilot signals are not used for transmission.    Patent Literature 1: JP-A-2008-172377 (Transmission device, receiving device, and method used in a mobile communication system using an OFDM system)    Non Patent Literature 1: 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8), 3GPP TS 36.211 V8.3.0, May of 2008, 6. 10 Reference Signals